This invention relates to a length-adjustable structural unit, which comprises a carrying part having a bearing surface, an extension part which is extensible on said bearing surface, and constraining means for preventing a tilting of the extension part even when it is extended. The constraining means comprise two flexible tensile elements that are disposed on opposite sides of the extension part. Each of said tensible elements are held in tension between the two ends of the bearing surface of the carrying part and are trained around at least two deflecting rollers, which are mounted on the extension part and are spaced apart at right angles to the bearing surface. The courses of the tension elements are disposed between the points where they are supported on the carrying part and those deflecting rollers which are nearest to said supporting points along the respective tensile element in a direction substantially parallel to the direction in which the extension part is extensible.
Structural units having a carrying part and an extension part, which is extensible from said carrying part, can be used for various purposes. The extension part may be used as a drawer or, for instance, as a slipform for use in bridge building or as an extensible crane boom. Length-adjustable structural units may be used also as shipping containers, slidable roofs or length-adjustable vehicles. It is desired within the scope of the invention to provide a structural unit which comprises a carrying part and an extension part which can be extended out of the carrying part and is guided in such a manner that forces acting transversely to the direction in which the extension part is extensible can be transmitted with simple means from the extension part to the carrying part.
To provide an arrangement comprising a carrying part and an extension part, which can be extended to a large extent out of the carrying part, it is known to guide the extension part on a bearing surface consisting, e.g., of two lateral rails and to provide the carrying portion with two tensioned ropes which extend along the bearing surface and in Z-shaped configurations are trained around two pulleys, which are mounted in the extension part. The load which is due to the weight of the extension part and tends to tilt the latter acts in the two lateral rope poritons in a sense tending to increase the size of the Z-shaped loop formed by the two ropes so that these two ropes can be stressed only in tension by the extension part. Because the loop length is predetermined by the lengths of the ropes, the Z-shaped loop formed by the ropes cannot increase so that a tilting of the extension part is automatically prevented. That arrangement which is used mainly for drawers has the disadvantage that the extension part is held only against tilting caused by a load which is due to a weight but is not held against a lateral displacement or canting under the action of lateral forces which may be due, e.g., to a wind load.
For this reason it is an object of the invention so to improve a structural unit of the kind described first hereinbefore that the extension part is held not only against vertical tilting, but a simple transmission of lateral forces acting on the extension part to the carrying part, is also ensured.
This object is accomplished according to the invention in that at least two of the deflecting rollers around which each tensile element is trained are spaced apart in a direction which is parallel to the bearing surface of the extension part and transverse to the direction in which the extension part is extensible.
Because each of the two tensile elements is trained around pulleys which are spaced apart at right angles to the bearing surface of the extension part and parallel to that surface, the loop formed by each tensile element has two components of action, one of which is at right angles to the bearing surface and the other of which is parallel to the bearing surface. As a result, the loop formed by each tensile element can transmit forces which act at right angles to the bearing surface of the extension part from the extension part to the carrying part, as well as forces which act parallel to the bearing surface and transversely to the direction in which the extensible part is extensible. Because such loop formed by a tensile element must be loaded only in a sense tending to increase its size, each loop can ensure a support of the extension part only to one side. For this reason, two tensile elements are required, which act in opposition to each other as regards the transmission of lateral forces.
Each tensile element has two courses, which extend from respective points where the tensile element is supported by the carrying part to the deflecting rollers which are nearest to said respective points of support along the respective tensile element. If said courses of each tensile element lie on opposite sides of the extension part, a simple arrangement can be provided in which the loop formed by each tensile element extends also parallel to the bearing surface. Such configuration of a tensile element can be obtained with a minimum number of deflecting rollers.
To permit an extension of an extension part not only along a straight line, a preferred feature of the invention resides in that at least one tensile element is adjustable in length. The path along which the extension part is extensible will automatically be given a curvature if one of the two tensile elements is shortened (or increased in length).
An embodiment of the invention is illustrated by way of example on the drawing, which is a perspective view showing a structural unit according to the invention consisting of a carrying part and an extensible extension part.